Generally, it is well known that the quality of an electrical steel sheet, which is used as the core material for electrical machinery, apparatuses, and the like, depends on such factors as so-called watt loss, magnetic flux density, orientation ratio, and aging deterioration, and, therefore, these factors are very important.
Among the above-mentioned factors, the term "aging deterioration" signifies a phenomenon in which the watt loss of an electrical steel sheet gradually deteriorates when the sheet is used for an extended period of time. One of the main causes of such deterioration is considered to be the residual carbon in the sheet.
Therefore, the starting material from which a non-oriented silicon-steel sheet is produced preferably has a carbon content of 0.0030% or less, as is disclosed in Japanese Patent Application No. 54-4785 previously filed by the applicant and entitled "Process for Producing a Non-oriented Silicon-Steel Sheet Exhibiting a Low Aging Deterioration and an Excellent Surface Property".
Heretofore, it was extremely difficult to attain a controlled carbon content of 0.0030% or less at the starting-material, i.e., the slab, stage. That is, even when it was possible to control the carbon content of the molten steel so that it did not exceed the desired value, e.g., 0.0030%, when decarburization was carried out during the melting stage by blowing an inert gas into the molten steel during the vacuum-degassing procedure. In the case of the ingot casting process, the molten steel picks up carbon from the mold stool when the molten steel is poured into the ingot mold. The resultant ingot solidifies in the ingot mold with large segregation of carbon.
Therefore, it was a common practice after hot-rolling or cold-rolling to decarburization-anneal the hot-rolled strip or the cold rolled strip so as to reduce the carbon content of the sheet to a desired value, e.g., 0.0030% or less. This decarburization-annealing of a hot-rolled strip or a product strip is called solid-phase decarburization. Said solid-phase decarburization results in a remarkable increase in the production cost as compared with liquid-phase decarburization at the molten steel stage. Moreover, solid-phase decarburization involves another problem in that depending upon the decarburization-annealing atmosphere, the surface of the steel sheet is oxidized, resulting in deterioration of the magnetic properties of the resultant electrical steel sheet.
If the [C] content of molten steel could be controlled so that it does not exceed the value of 0.0030% in the course of the production of a slab from molten steel, the above-mentioned problems could be solved in one stroke.
Recently, a continuous-casting process has been established as a process for producing a steel slab. The production of steel for producing an electrical steel sheet (hereinafter referred to as steel for an electrical steel sheet) has also been carried out in series in a steelmaking furnace (e.g., a converter), a vacuum-degassing apparatus, and a continuous-casting machine. However steelmaking, in this method has created new problems. That is, the vacuum-degassing procedure involves a problem in that the desired [C] content of the steel must be ensured within as short a time as possible in order to compete with the operating capability of the subsequent continuous-casting procedure. In other words, the degassing time is apt to not be satisfactory enough to ensure a desired [C] content of the steel. Moreover, although the continuous-cast slab has not large segregation of carbon, as is the case in the steel ingot-casting process, it involves a problem in that the [C] content of the steel at the starting material, i.e., the stab stage exceeds the desired value, e.g., 0.0030%, irregardless of the fact that the desired [C] content was ensured in the degassing procedure.